New developments in the N-heterocyclic carbene chemistry: Application as organocatalysts

Abstract

N-heterocyclic carbenes have been studied for their abilities to catalyze C-C bond formation/cleavage and asymmetric reactions. N-heterocyclic carbenes generated from imidazolium, benzimidazolium, triazolium, pyrido[l,2-c] imidazolium, pyrido[2,1-c]triazolium, and dipyrido[1,2-c : 2′,1′-e] imidazolium iodide were found to be effective catalysts in benzoin condensation. The acidity of the azolium salts at the C2-hydrogen of the imidazolium or the triazolium moieties relates to the base effect on the reaction. These carbenes can also be employed in retro-benzoin condensation, which is a new method of synthesizing ketones by C-C bond cleavage of a-substituted benzoins. Chloro and fluoro groups of haloarenes are nucleophilically substituted by aroyl groups originated from aromatic aldehydes by catalytic action of imidazolidenyl and triazolidenyl carbenes to afford ketones. AT-heterocyclic carbenes mediate the addition of trimethylsilylcyanide to aldehydes to yield cyanohydrin trimethylsilyl ethers. The use of chiral imidazolidenyl carbene derived from (R,R)-1,3-bis[(1-naphthyl)ethyl]imidazolium chloride led to enantioseletive cyanosilylation. C2-symmetric imidazolidenyl carbenes catalyze asymmetric acylation of racemic secondary alcohols.